While implantable ICDs frequently deliver life saving therapy, occasionally an unnecessary electrical shock can be delivered to a patient's heart in response to rapid heart rates caused by exercise (e.g. sinus tachycardia) or by atrial fibrillation. Such rhythms, known collectively as supraventricular tachycardias (SVTs), may occur in up to 30% of ICD patients.
Anti-tachycardia pacing (ATP), a painless therapy, can be used to substantially terminate many monomorphic VTs without delivering unnecessary electrical shocks. While ATP is painless, ATP may not deliver effective therapy for all types of VTs. For example, ATP may not be as effective for polymorphic VTs, which is a fast rhythm (VTs) with variable morphologies. Polymorphic VTs and ventricular fibrillation (VFs) can be more lethal and require expeditious treatment by shock. The morphology of the QRS complex in the electrogram (EGM) signal may be used to discriminate a SVT episode from a VT episode or a monomorphic VT episode from a polymorphic VT or VF episode. Polymorphic VT and VF episodes have similar EGM morphology characteristics, and thus will be referred to interchangeably in this application. In the first case, the EGM morphology of each beat of an episode is compared to the morphology of a sample waveform recorded from the normal heartbeat, typically referred to as the template. In the second case, the morphology of each beat of a VT episode may be compared to that of one or more different beats from the same episode, which serve(s) as the template(s). One morphological method to discriminate between an episode beat and a template beat based on wavelet comparison. A template beat is either a normal beat or another beat from the same episode. An exemplary wavelet comparison method may be seen with respect to U.S. Pat. No. 6,393,316 issued May 21, 2002, and assigned to the assignee of the present disclosure. Generally, the wavelet comparison method involves aligning the EGM signal with the template signal based on certain characteristics (eg. peaks or valleys), transforming the digitized signal into signal wavelet coefficients, then identifying higher amplitude digitized signals of the signal wavelet coefficients. Thereafter, a match metric is generated that corresponds to the higher amplitude digitized signals of the signal wavelet coefficients. A corresponding set of template wavelet coefficients is derived from signals indicative of a heart depolarization of known type.
While the wavelet comparison method successfully eliminates or substantially reduces unnecessary electrical shocks delivered to a patient's heart, the wavelet comparison method requires shifting and alignment of the episode beat to the template beat for accurate morphologic discrimination. Shifting and alignment of the episode beat to the template beat may be computationally expensive, especially for monomorphic versus polymorphic VT discrimination in which more than one template beats derived from the same episode may be needed for comparison. Other template matching methods known in the art such as correlation, area of difference, etc. that could be used in place of the wavelet method are also computationally expensive. Thus, it may be beneficial to develop additional or alternative methods that are able to distinguish SVT from VT or monomorphic VT from polymorphic VT.